‘Anomalously cold’ system forecast to hit Northern California

The system is expected to bring chances of rain to some locations and below-normal temperatures across the region Friday and over the three-day Labor Day weekend, when many Californians head to the beach and mountain lakes. 

The National Weather Service issued a special statement for portions of far northern and northeastern California, including the greater Tahoe area, Markleeville and Mammoth Lakes: “If you have outdoor recreation plans this holiday weekend, be sure that you are prepared for the cold, wet, and windy conditions.” 

The system is forecast to disrupt the heat event, which sent temperatures soaring this week, with a blast of cold air. Temperatures across the state were well above normal Wednesday, with inland areas in the 90s and 100s, and by Friday, they will likely be below normal, up to 20 degrees below normal in the most extreme cases. Sacramento hit a high of 95 degrees Wednesday, while the forecast high Friday is 76. San Francisco reached 83 degrees Wednesday; the high Friday is predicted to be 68 degrees. Tahoe City soared to 86 Wednesday; it’s expected to reach only 61 Friday. Some might call it weather whiplash. 

“We’re basically going below normal for Friday, Saturday and Sunday — that’s across California,” said Bryan Jackson, a meteorologist with the Weather Prediction Center. Monday is likely to be a little warmer than Friday through Sunday, he said.

The abrupt shift is forecast to come as a trough of low pressure that began developing off the West Coast on Thursday absorbs a larger low-pressure system that is expected to dive down into California from western Canada and the Pacific Northwest on Friday and Saturday.

“It’s bringing a push of moisture and triggering storms,” said Katrina Hand, a forecaster with the weather service’s Sacramento office.

“An unseasonably strong low pressure system will bring cooler temperatures, breezy onshore winds and increased shower and thunderstorm chances into the weekend,” read the forecast from the Sacramento office, which monitors the weather in the northern Sacramento Valley, Sierra foothills and Shasta area. 

The impact on the San Francisco Bay Area is expected to be less dramatic. “Cooling temperatures this weekend with small chances of drizzle as low pressure aloft establishes itself nearby,” the forecast from the NWS Bay Area office said.

The system will bring a chance of rain to many parts of Northern California, especially mountainous areas, including the northern Sierra Nevada and the Cascade, Klamath and Siskiyou ranges.

Some of the highest potential for storm activity in California is over the “southern Cascades and northern Sierra from Butte and Plumas counties north into Eastern Shasta county,” the weather service said. The main impacts in these areas are likely to be “small hail, frequent lightning and brief heavy rain.”

To the west of Shasta, Trinity County also has a significant chance of rain. The weather service’s Eureka office said locations within the county could see close to an inch of rain over 48 hours. In far northern California, rainfall amounts on the coast are likely to be much lower compared with mountain areas, up to a tenth of an inch, and rain in Mendocino and Lake counties is forecast to be “minimal,” the weather service said. 

Jackson said the Smith River Complex, burning at the California-Oregon border, is likely to see rain. The wildfire, covering over 75,000 acres, contributed to the smoky conditions in the Bay Area on Wednesday. “It’s beneficial rain for up there,” he said.

In the Lake Tahoe Basin, there’s a 20% to 40% chance of rain and a 10% to 20% chance of thunderstorms on Friday, the weather service said. Rain chances continue on Saturday (a roughly 20% to 45% chance) and into Sunday (40% to 50%).

The rain is expected to mostly be light and scattered. Storm totals for the Tahoe Basin, Friday through Sunday, are expected to range from a quarter to a half inch, with higher amounts at higher elevations. Areas around the lake could record about a quarter inch. “If there are any thunderstorms that stick around for longer, we could see locally higher amounts,” Hand said. 

She added, “With this system, the majority of the precipitation is looking like it’s going to fall in far northern California — the coastal range, southern Cascades and Shasta County — but we are still looking at decent precipitation chances for the northern Sierra as well,” Hand said.  

In the Sacramento Valley, Hand said the chance of showers is slight, though it’s a bit higher in the upper parts of the valley near Redding.

In the San Francisco Bay Area, the likelihood of rain is very slim. “The chances are mainly along the coast, where we could get some drizzle,” said Dalton Behringer, a forecaster with the weather service’s Bay Area office. 

The Bay Area is forecast to see a big cool down Friday, with locations on the coast and along the bay front likely to be in the 60s and inland areas in the 70s. Temperatures on Saturday and Sunday are expected to be similar, though a tad warmer. “The coldest day will likely be Friday, but for places along the bay shoreline, there’s not going to be much change Friday through Sunday,” Behringer said.

Monday is likely to mark the start of a warmup in the Bay Area, with inland locations forecast to be in the 80s.